Brian often appears at the press nights.
The press night may or may not be the first show. A number of the early shows in each place may be classed as previews.
In the case of Manchester the 1st night was 20th March but the big opening press night was not until the 24th (when Brian/Roger and a few of their and the casts celeb friends turned up).
Not sure if there are any previews for Sunderland but as its quite a short run (only 2 1/2 weeks) compared to the rest they may do the press night on the 1st night.